Drop mpark variant and require std::variant

As and when std::variant is universally available, look at dropping mpark variant fallback.

  • CentOS 8
  • MacOS 11.0
  • MacOS 10.15
  • MacOS 10.14
  • Ubuntu 20.04
  • FreeBSD 12.2
  • VS2019
  • VS2017
Edited by Roger Leigh
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